Ferric iron content of mineral inclusions in diamonds from George Creek, Colorado determined using Mössbauer spectroscopy

The Mössbauer milliprobe allows the determination of Fe³⁺/ΣFe in samples as small as 50?μm. For the first time this technique is applied to a suite of diamonds of eclogitic paragenesis, where three garnet and five clinopyroxene inclusions in diamonds from George Creek, Colorado have been analysed. For garnet Fe³⁺/ΣFe ranges from 0–7%, while values for clinopyroxene range from 8–14%. These results are consistent with the low oxygen fugacity conditions implied by the presence of the inclusions in diamond.     

Peculiarities of Diamonds in Eclogitic Xenoliths from the Komsomolskaya Kimberlite Pipe,Yakutia

The first studies of diamonds in eclogitic xenoliths from the Komsomolskaya kimberlite pipe are described. Among round and oval-shaped xenoliths with diamond ingrowths, samples with a garnet content of 40–90% of the xenolith volume dominate. Two eclogite samples contain grains of accessory rutile; a kyanite sample is also revealed. Certain samples contain two or more crystals of diamonds. Diamonds with an octahedral habit and crystals with transitional habits, which belong to an octahedral-rhombic dodecahedral row, dominate in eclogites; there are many variety VIII aggregates. A high concentration of structural nitrogen, commonly in the A form, was registered in most of the crystals. Diamonds with a small content of nitrogen impurities, 40–67% in the B1 form, are present in a number of xenoliths. The calculated temperatures of the formation of eclogitic xenoliths is 1100–1300°C. Diversity in the impurity compositions of diamonds in the same xenolith shows that these diamonds were formed at various times and in different settings. The diamond position in xenoliths, the various level of nitrogen aggregation in the diamonds, and a number of other factors point to the later formation of the diamonds, as compared to minerals of eclogites, from fluid or fluid-melts in the process of metasomatosis.

     

Trace elements in garnets and chromites: Diamond formation in the Siberian lithosphere

Proton-microprobe analyses of trace elements in garnet and chromite inclusions in diamonds (DI) from the Mir, Udachnaya, Aikhal and Sytykanskaya kimberlites in Yakutia, CIS, provide new insights into the processes that form diamond. Equivalent data on garnet and chromite concentrates from these pipes yield information on the thermal state and chemical stratification of the Siberian lithosphere. Peridotite-suite diamonds from Yakutia have formed over a temperature interval of ca. 600°C, as measured by Ni and Zn thermometry on garnet and chromite inclusions in diamonds. Individual diamonds contain inclusions recording temperature intervals of >400°C; ranges of >100°C are common. Diamond formation followed a severe depletion event(s), and a separate enrichment in Sr. Comparison of temperatures on DI garnet and spinel with temperatures derived from diamondiferous harzburgites, exposed inclusions in boart and concentrate minerals suggests that the diamond-containing part of the lithosphere has cooled significantly since the Siberian diamonds crystallized. The peridotite-suite diamonds probably formed mainly in response to one or more relatively short-lived thermal events, related to magmatic intrusion. The northern part of the Daldyn-Alakit district may have had a typical cratonic geotherm at the time of diamond formation, and during kimberlite intrusion. The southern part of the district, and the Malo-Botuobiya kimberlite field, probably had a relatively low geotherm (ca. 35 mW/m²). The vertical distribution of garnet and chromite types indicates that the mantle above 120 km depth is dominated by lherzolites, whereas the deeper parts of the lithosphere are a mixture of lherzolites and more depleted harzburgites and dunites.     

Enrichment processes at the base of the Archean lithospheric mantle: observations from trace element characteristics of pyropic garnet inclusions in diamonds

Trace element concentrations of peridotitic garnet inclusions in diamonds from two Chinese kimberlite pipes were determined using the ion microprobe. Garnet xenocrysts from the same two kimberlite pipes were also analyzed for comparison. In contrast to their extremely refractory major element compositions, all harzburgitic garnets showed enrichment in light rare earth elements (REE) relative to chondrite, resulting in sinuous REE patterns. Both normal and sinuous REE patterns were observed from the lherzolitic garnets. Concentrations of REE in garnets changed significantly from diamond to diamond and no specific correlations were observed with their major element compositions. Analyses of randomly selected two to three points within every grain of a large number of garnet inclusions by the ion microprobe demonstrated that there was no evident compositional heterogeneity, and multiple grains of one phase from a single diamond host also exhibit very similar compositions. This implies that the trace element heterogeneity within one grain or among multiple inclusions from the same diamond host, as reported from Siberian diamonds, is not a common feature for these Chinese diamonds. Concentrations of Na, Ti, and Zr tend to decrease when garnets become more refractory, but variations of Sr and Li are more complex. Compositions rich in light REE and relatively poor in high field strength elements (HFSE) of the harzburgitic garnet inclusions in diamonds are generally consistent with metasomatism by carbonatite melts. The trace element features observed from the garnet inclusions in Chinese diamonds may be caused by carbonatite melt infiltration and partial melt extraction. Spatial and temporal gradients in melt/rock ratio and temperature are the main reasons for the large variations of REE patterns and other trace element concentrations. Received: 27 April 1999 / Accepted: 1 March 2000     

Diamondiferous garnet harzburgites from the Finsch kimberlite,Northern Cape,South Africa

Two xenoliths of garnet harzburgite from the Finsch kimberlite, South Africa, have been found to contain diamond. One of the xenoliths has mineral compositions typical of low-T coarse textured garned peridotites, whereas minerals in the other are similar but not identical to most peridotite-suite minerals included in diamonds, especially in the low-CaO content of garnet. Geothermobarometric calculations show both xenoliths equilibrated at temperatures above 1,100°C and pressures>55 kbar, which is near the low-pressure end of the range of equilibration conditions for diamond-free garnet lherzolites and garnet harzburgites from Finsch. The chemistries of the minerals in the two rocks are distinctly different to most of the mineral inclusions in Finsch diamonds. This, as well as the different ¹³C compositions between xenolith diamonds (-2.8 to-4.6) and diamonds in the kimberlite (generally<-4.3) suggest different origins or sources for the diamonds.     

A study of eclogitic diamonds and their inclusions from the Finsch kimberlite pipe, South Africa

Previous studies of diamonds from Finsch have shown that eclogitic inclusions are rare at Finsch and that the eclogitic garnet and clinopyroxenes are iron and manganese-rich. In order to expand the current database of information, 93 eclogitic diamonds were selected for this study. Eight diamonds were polished into plates for cathodoluminescence studies and infrared examination of diamond growth and 31 diamonds were cracked to retrieve inclusions. The eclogitic garnets analysed in this study are enriched in Fe and are relatively depleted in Ca and Mg relative to worldwide data. FeO contents for garnet range from 15 to 27 wt.% and MnO contents reach a maximum value of 1.6 wt.%. The eclogitic clinopyroxenes have relatively high FeO contents, up to 14.8 wt.% and K₂O contents are low (<0.4 wt.%). Three non-touching garnet–clinopyroxene mineral pairs produce equilibration temperatures of 1138–1179 °C at an assumed pressure of 50 kb. No Type II diamonds were found during this study, all diamonds are of Type IaAB. Total nitrogen contents of Type IaAB diamonds range from 11 to 1520 ppm, with variable aggregation states (up to 84% nitrogen aggregated as B-defects). Distinct infrared characteristics suggest that the Finsch kimberlite sampled either more than one mantle source region of similar age but differing temperature, or two different populations of diamonds with different ages. The diamonds provide evidence of changing mantle conditions during crystallisation. Continuous diamond growth is illustrated by the presence of regular octahedral growth zones, although in some diamonds cubic growth is noted. One diamond shows evidence of platelet degradation, suggesting exposure to high temperatures and/or shearing stresses.     

A mineralogical Raman spectroscopy study on eclogitic garnet inclusions in diamonds from Argyle

In an attempt to better define the depths of formation of eclogitic-paragenesis diamonds from the Argyle lamproite pipe, we have employed a Laser Raman microprobe to determine the Raman peak shift of a garnet inclusion (extracted from diamond) with pressure in a diamond-anvil pressure cell. On the basis of these data, we further found that the in situ garnet inclusions record near-atmospheric pressures within the limits of experimental uncertainty. Data on the compressibility and thermal expansivity of both diamond and garnet were used to define a P-T curve for the entrapment of garnet in diamond. A window within the range 47 kbar at 1100° C (150 km) to 93 kbar at 1500° C (280 km) for the formation of syngenetic garnet inclusions in diamond is defined by the intersection of the continental geotherm with the diamond-graphite boundary and the entrapment curve determined in the present study. This P-T window is consistent with the constraints imposed by other petrological studies of co-existing inclusions. Most of eclogitic-paragenesis diamonds from Argyle are estimated to have formed at a depth less than 250 km, if temperature estimates from petrological study are used.     

Mineral inclusions in fibrous diamonds: constraints on cratonic mantle refertilization and diamond formation

We analyzed mineral microinclusions in fibrous diamonds from the Wawa metaconglomerate (Superior craton) and Diavik kimberlites (Slave craton) and compared them with published compositions of large mineral inclusions in non-fibrous diamonds from these localities. The comparison, together with similar datasets available for Ekati and Koffiefontein kimberlites, suggest a general pattern of metasomatic alteration imposed on the ambient mantle by formation of fibrous diamond. Calcium and Fe enrichment of peridotitic garnet and pyroxenes and Fe enrichment of olivine associated with fibrous diamond-forming fluids contributes to refertilization of the cratonic mantle. Saline—carbonatitic—silicic fluid trapped by fibrous diamonds may represent one of the elusive agents of mantle refertilization. Calcium enrichment of peridotitic garnet and pyroxenes is expected in local mantle segments during fibrous diamond production, as Ca in the carbonatitic fluids is deposited into the surrounding mantle when oxidized carbon is reduced to diamond. Harzburgitic garnet evolves towards Ca-rich compositions even when it interacts with Ca-poor saline fluids. An unusual trend of Mg enrichment to Fo_95–98 is observed in some olivine inclusions in Wawa fibrous diamonds. The trend may result from the carbonatitic composition of the fluid that promotes crystallization of magnesian olivine and preferentially oxidizes the fayalite component. We propose a generic model of fibrous and non-fibrous diamond formation from carbonatitic fluids that explains enrichment of the mantle in mafic magmaphile and incompatible elements and accounts for locally metasomatized compositions of diamond inclusions.     

Inclusions in diamonds from Snap Lake kimberlites (Slave Craton,Canada): Geochemical features of crystallization

The results of integrated studies of inclusion-containing diamonds from kimberlites of the Snap Lake dike complex (Canada) are presented. Features of the morphology, defect–impurity composition, and internal structure of the diamonds were determined by optic and scanning microscopy. The chemical composition of crystalline inclusions (olivine, garnet, and pyroxene) in diamonds was studied using a microanalyzer with an electronic probe. The inclusions of ultramafic paragenesis in the diamond (87%) are predominant. Carbonates, sulfide and hydrated silicate phases were found only in multiphase microinclusions. The large phlogopite inclusion studied was similar in composition to earlier studied nanosize inclusions of high-silica mica in diamonds from Snap Lake kimberlites. Revealed features of studied diamonds and presence of high-silica mica suggest that diamonds from Snap Lake have formed as the result of interaction between enriched in volatile and titanium high-potassium carbonate–silicate melts and peridotitic substrate at the base of thick lithospheric mantle.     

Mineral inclusions in diamonds from the Sputnik kimberlite pipe, Yakutia

The Sputnik kimberlite pipe is a small “satellite” of the larger Mir pipe in central Yakutia (Sakha), Russia. Study of 38 large diamonds (0.7-4.9 carats) showed that nine contain inclusions of the eclogitic paragenesis, while the remainder contain inclusions of the peridotitic paragenesis, or of uncertain paragenesis. The peridotitic inclusion suite comprises olivine, enstatite, Cr-diopside, chromite, Cr-pyrope garnet (both lherzolitic and harzburgitic), ilmenite, Ni-rich sulfide and a Ti-Cr-Fe-Mg-Sr-K phase of the lindsleyite-mathiasite (LIMA) series. The eclogitic inclusion suite comprises omphacite, garnet, Ni-poor sulfide, phlogopite and rutile. Peridotitic ilmenite inclusions have high Mg, Cr and Ni contents and high Nb/Zr ratios; they may be related to metasomatic ilmenites known from peridotite xenoliths in kimberlite. Eclogitic phlogopite is intergrown with omphacite, coexists with garnet, and has an unusually high TiO₂ content. Comparison with inclusions in diamonds from Mir shows general similarities, but differences in details of trace-element patterns. Large compositional variations among inclusions of one phase (olivine, garnet, chromite) within single diamonds indicate that the chemical environment of diamond crystallisation changed rapidly relative to diamond growth rates in many cases. P-T conditions of formation were calculated from multiphase inclusions and from trace element geothermobarometry of single inclusions. The geotherm at the time of diamond formation was near a 35 mW/m² conductive model; that is indistinguishable from the Paleozoic geotherm derived by studies of xenoliths and concentrate minerals from Mir. A range of Ni temperatures between garnet inclusions in single diamonds from both Mir and Sputnik suggests that many of the diamonds grew during thermal events affecting a relatively narrow depth range of the lithosphere, within the diamond stability field. The minor differences between inclusions in Mir and Sputnik may reflect lateral heterogeneity in the upper mantle.     

Two diamond-bearing peridotite xenoliths from the finsch kimberlite,South Africa

Two diamond bearing xenoliths found at Finsch Mine are coarse garnet lherzolites, texturally and chemically similar to the dominant mantle xenoliths in that kimberlite. A total of 46 diamonds weighing 0.053 carats have been recovered from one and 53 diamonds weighing 0.332 carats from the other. The diamonds are less corroded than diamonds recovered from the kimberlite. Geothermobarometric calculations indicate that the xenoliths equilibrated at 1,130° C and pressures 50 kb which is within the diamond stability field; this corresponds to depths of 160 km and would place the rocks on a shield geotherm at slightly greater depths than most coarse garnet lherzolites from kimberlite. The primary minerals in the two rocks are very similar to each other but distinctly different to the majority of mineral inclusions in Finsch diamonds. This suggests a different origin for the diamonds in the kimberlite and the diamonds in the xenoliths although the equilibration conditions for both suites are approximately coincident and close to the wet peridotite solidus.     

Diamonds from the Buffalo Head Hills,Alberta: Formation in a non-conventional setting

Kimberlite pipes K11, K91 and K252 in the Buffalo Head Hills, northern Alberta show an unusually large abundance (20%) of Type II (no detectable nitrogen) diamonds. Type I diamonds range in nitrogen content from 6 ppm to 3300 ppm and in aggregation states from low (IaA) to complete (IaB). The Type IaB diamonds extend to the lowest nitrogen concentrations yet observed at such high aggregation states, implying that mantle residence occurred at temperatures well above normal lithospheric conditions. Syngenetic mineral inclusions indicate lherzolitic, harzburgitic, wehrlitic and eclogitic sources. Pyropic garnet and forsteritic olivine characterize the peridotitic paragenesis from these pipes. One lherzolitic garnet inclusion has a moderately majoritic composition indicating a formation depth of ∼ 400 km. A wehrlitic paragenesis is documented by a Ca-rich, high-chromium garnet and very CaO-rich (0.11–0.14 wt.%) olivine. Omphacitic pyroxene and almandine-rich garnet are characteristic of the eclogitic paragenesis. A bimodal δ¹³C distribution with peaks at − 5‰ and − 17‰ is observed for diamonds from all three kimberlite pipes. A large proportion (∼ 40%) of isotopically light diamonds (δ¹³C < −10‰) indicates a predominantly eclogitic paragenesis.The Buffalo Head Terrane is of Lower Proterozoic metamorphic age (2.3–2.0 Ga) and hence an unconventional setting for diamond exploration. Buffalo Hills diamonds formed during multiple events in an atypical mantle setting. The presence of majorite and abundance of Type II and Type IaB diamonds suggests formation under sublithospheric conditions, possibly in a subducting slab and resulting megalith. Type IaA to IaAB diamonds indicate formation and storage under lower temperature in normal lithospheric conditions.     

Ar–Ar age determinations of eclogitic clinopyroxene and garnet inclusions in diamonds from the Venetia and Orapa kimberlites

Ar–Ar age measurements are reported for selected eclogitic clinopyroxene and garnet inclusions in Orapa diamonds and clinopyroxene inclusions in Venetia diamonds. Laser drilling of encapsulated clinopyroxene inclusions within Venetia diamonds released a maximum of 3% of the total ⁴⁰Ar, indicating little diffusive transfer and storage of radiogenic ⁴⁰Ar at the diamond–inclusion boundary. Apparent ages obtained during stepped heating of three diamonds are consistent with diamond crystallisation occurring just prior to the kimberlite eruption 520 Ma ago. Stepped heating of three clinopyroxene-bearing Orapa diamonds gave ages of 906–1032 Ma, significantly above the eruption age, but consistent with previously determined isotopic ages. A few higher apparent ages hint at the presence an older generation of Orapa diamonds that formed >2500 Ma ago. Orapa garnets also contain measurable K contents, and record a range of ages between 1000 and 2500 Ma. The old apparent ages and lack of significant interface ⁴⁰Ar released by the laser probe, suggests that pre-eruption radiogenic ⁴⁰Ar and mantle-derived ⁴⁰Ar components are trapped in microinclusions within the pyroxene and garnet inclusions.     

Petrology of two diamondiferous eclogite xenoliths from the Lahtojoki kimberlite pipe, eastern Finland

Diamondiferous Group A eclogites constitute a minor portion of the mantle-derived xenoliths in the eastern Finland kimberlites. They have been derived from the depth interval 150–230 km where they are inferred to occur as thin layers or small pods within coarse-grained garnet peridotites. The chemical and isotopic composition of minerals suggest that they represent (Proterozoic?) mantle-derived melts or cumulates rather than subducted oceanic lithosphere. During magma ascent and emplacement of the kimberlites, the eclogite xenoliths were mechanically and chemically rounded judging from the types of surface markings. In addition, those octahedral crystal faces of diamonds that were partially exposed from the rounded eclogite xenolith became covered by trigons and overlain by microlamination due to their reaction with the kimberlite magma. The diamonds bear evidence of pervasive plastic deformation which is not, however, evident in the eclogite host. This suggests that annealing at ambient lithospheric temperatures has effectively recrystallised the silicates while the diamond has retained its lattice imperfections and thus still has the potential to yield information about ancient mantle deformation. One of our samples is estimated to contain approximately 90,000 ct/ton diamond implying that some diamonds occur within very high-grade pods or thin seams in the lithospheric mantle. To our knowledge, this is one of the most diamondiferous samples described.     

The formation of peridotitic suite inclusions in diamonds

Olivine, orthopyroxene and garnet grains belonging to the peridotitic suite of mineral inclusions in natural diamonds typically show compositions poorer in Ca and Al and richer in Mg and Cr than the same minerals in peridotite nodules in kimberlite. Other features suggest the crystallisation of diamonds from magmas of kimberlitic affinities, and it is suggested that the genesis of peridotitic suite diamonds is linked with that of a CO₂-bearing magma. It is shown that the generation of kimberlitic magma from common garnet-peridotite (with 5 wt.% clinopyroxene) in the presence of CO₂ may rapidly remove by melting all Ca-rich solid phases (clinopyroxene and/or carbonate). Further melting may form liquids in equilibrium with olivine, orthopyroxene, and garnet with the distinctive compositions of the diamond inclusions. The amount of melting and CO₂ necessary for the loss of clinopyroxene (and/or carbonate) are estimated at approximately 5.0 wt.% and 0.5 wt.% respectively.     

Aspects of diamond mineralisation and distribution at the Helam Mine, South Africa

The diamonds from the Swartruggens dyke swarm are mainly tetrahexahedra, with subsidiary octahedral and cuboid crystals. They are predominantly colourless, with subordinate yellows, browns, and greens. The existence of discrete cores and oscillatory growth structures within the diamonds, together with the recognition of harzburgite, lherzolite, at least two eclogitic and a websteritic diamond paragenesis, variable nitrogen contents, and both Type IaAB and Type Ib–IaA diamonds provides evidence for episodic diamond growth in at least six different environments. The predominance of plastic deformation in the diamonds, the state of nitrogen aggregation, and the suite of inclusion minerals recovered are all consistent with a xenocrystic origin for the diamonds, with the Type Ib–IaA diamonds being much younger than the rest. Mantle storage at a time-averaged temperature of ±1100 °C is inferred for the Type IaAB diamonds. The distribution of mantle xenocrysts of garnet and chromite within the high-grade Main kimberlite dyke compared to the low-grade Changehouse kimberlite dyke strongly suggests that the difference in diamond content is due to an increased eclogitic component of diamonds in the Main kimberlite dyke.     

The trace element composition of silicate inclusions in diamonds: a review

On a global scale, peridotitic garnet inclusions in diamonds from the subcratonic lithosphere indicate an evolution from strongly sinusoidal REE_N, typical for harzburgitic garnets, to mildly sinusoidal or “normal” patterns (positive slope from LREE_N to MREE_N, fairly flat MREE_N–HREE_N), typical for lherzolitic garnets. Using the Cr-number of garnet as a proxy for the bulk rock major element composition it becomes apparent that strong LREE enrichment in garnet is restricted to highly depleted lithologies, whereas flat or positive LREE–MREE slopes are limited to less depleted rocks. For lherzolitic garnet inclusions, there is a positive relation between equilibration temperature, enrichment in MREE, HREE and other HFSE (Ti, Zr, Y), and decreasing depletion in major elements. For harzburgitic garnets, relations are not linear, but it appears that lherzolite style enrichment in MREE–HREE only occurs at temperatures above 1150–1200 °C, whereas strong enrichment in Sr is absent at these high temperatures. These observations suggest a transition from melt metasomatism (typical for the lherzolitic sources) characterized by fairly unfractionated trace and major element compositions to metasomatism by CHO fluids carrying primarily incompatible trace elements. Melt and fluid metasomatism are viewed as a compositional continuum, with residual CHO fluids resulting from primary silicate or carbonate melts in the course of fractional crystallization and equilibration with lithospheric host rocks.

Eclogitic garnet inclusions show “normal” REE_N patterns, with LREE at about 1× and HREE at about 30× chondritic abundance. Clinopyroxenes approximately mirror the garnet patterns, being enriched in LREE and having chondritic HREE abundances. Positive and negative Eu anomalies are observed for both garnet and clinopyroxene inclusions. Such anomalies are strong evidence for crustal precursors for the eclogitic diamond sources. The trace element composition of an “average eclogitic diamond source” based on garnet and clinopyroxene inclusions is consistent with derivation from former oceanic crust that lost about 10% of a partial melt in the garnet stability field and that subsequently experienced only minor reenrichment in the most incompatible trace elements. Based on individual diamonds, this simplistic picture becomes more complex, with evidence for both strong enrichment and depletion in LREE.

Trace element data for sublithospheric inclusions in diamonds are less abundant. REE in majoritic garnets indicate source compositions that range from being similar to lithospheric eclogitic sources to strongly LREE enriched. Lower mantle sources, assessed based on CaSi–perovskite as the principal host for REE, are not primitive in composition but show moderate to strong LREE enrichment. The bulk rock LREE_N–HREE_N slope cannot be determined from CaSi–perovskites alone, as garnet may be present in these shallow lower mantle sources and then would act as an important host for HREE. Positive and negative Eu anomalies are widespread in CaSi–perovskites and negative anomalies have also been observed for a majoritic garnet and a coexisting clinopyroxene inclusion. This suggests that sublithospheric diamond sources may be linked to old oceanic slabs, possibly because only former crustal rocks can provide the redox gradients necessary for diamond precipitation in an otherwise reduced sublithospheric mantle.     

Origin of sub-lithospheric diamonds from the Juina-5 kimberlite (Brazil): constraints from carbon isotopes and inclusion compositions

Forty-one diamonds sourced from the Juina-5 kimberlite pipe in Southern Brazil, which contain optically identifiable inclusions, have been studied using an integrated approach. The diamonds contain <20 ppm nitrogen (N) that is fully aggregated as B centres. Internal structures in several diamonds revealed using cathodoluminescence (CL) are unlike those normally observed in lithospheric samples. The majority of the diamonds are composed of isotopically light carbon, and the collection has a unimodal distribution heavily skewed towards δ¹³C ~ ?25 ‰. Individual diamonds can display large carbon isotope heterogeneity of up to ~15 ‰ and predominantly have isotopically lighter cores displaying blue CL, and heavier rims with green CL. The light carbon isotopic compositions are interpreted as evidence of diamond growth from abiotic organic carbon added to the oceanic crust during hydrothermal alteration. The bulk isotopic composition of the oceanic crust, carbonates plus organics, is equal to the composition of mantle carbon (?5 ‰), and we suggest that recycling/mixing of subducted material will replenish this reservoir over geological time. Several exposed, syngenetic inclusions have bulk compositions consistent with former eclogitic magnesium silicate perovskite, calcium silicate perovskite and NAL or CF phases that have re-equilibrated during their exhumation to the surface. There are multiple occurrences of majoritic garnet with pyroxene exsolution, coesite with and without kyanite exsolution, clinopyroxene, Fe or Fe-carbide and sulphide minerals alongside single occurrences of olivine and ferropericlase. As a group, the inclusions have eclogitic affinity and provide evidence for diamond formation at pressures extending to Earth’s deep transition zone and possibly the lower mantle. It is observed that the major element composition of inclusions and isotopic compositions of host Juina-5 diamonds are not correlated. The diamond and inclusion compositions are intimately related to subducted material and record a polybaric growth history across a depth interval stretching from the lower mantle to the base of the lithosphere. It is suggested that the interaction of slab-derived melts and mantle material combined with subsequent upward transport in channelised networks or a buoyant diapir explains the formation of Juina-5 diamonds. We conclude that these samples, despite originating at great mantle depths, do not provide direct information about the ambient mantle, instead, providing a snapshot of the Earth’s deep carbon cycle.     

Diamonds and eclogites of the Jericho kimberlite (Northern Canada)

We studied diamonds and barren and diamondiferous eclogite xenoliths from the Jericho kimberlite (Northern Slave craton). The majority of the diamonds are non-resorbed octahedral crystals, with moderately aggregated N (IaB < 50%, N < 300 ppm) and δ¹³C = −5 to −41‰. The diamonds belong to “eclogitic” (90% of the studied samples), “websteritic” (7%) and “peridotitic” (3%) assemblages. The Jericho diamonds differ from the majority of “eclogitic” diamonds worldwide in magnesian compositions of associated minerals and extremely light C isotopic compositions (δ¹³C = −24 to −41‰). We propose that metasomatism triggered by H₂O fluids may have been involved in the diamond formation. Multiple episodes of the metasomatism and associated melt extraction of various ages are evident in Jericho eclogite xenoliths where primary garnet and clinopyroxene have been recrystallized to more magnesian minerals with higher contents of some incompatible trace elements and to hydrous secondary phases. The model is supported by the general similarity of mineral compositions in diamondiferous eclogites to those in diamond inclusions and to secondary magnesian garnet and clinopyroxene in recrystallized barren eclogites. The ultimate products of the metasomatism could be “websteritic” diamond assemblages sourced from magnesian eclogites. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Episodic diamond genesis at Jwaneng, Botswana, and implications for Kaapvaal craton evolution

Major element and Re–Os isotope analysis of single sulfide inclusions in diamonds from the 240 Ma Jwaneng kimberlite has revealed the presence of at least two generations of eclogitic diamonds at this locality, one Proterozoic (ca. 1.5 Ga) and the other late Archean (ca. 2.9 Ga). The former generation is considered to be the same as that of eclogitic garnet and clinopyroxene inclusion bearing diamonds from Jwaneng with a Sm–Nd isochron age of 1.54 Ga. The latter is coeval with the 2.89 Ga subduction-related generation of eclogitic sulfide inclusion bearing diamonds from Kimberley formed during amalgamation of the western and eastern Kaapvaal craton near the Colesberg magnetic lineament.

The Kimberley, Jwaneng, and Premier kimberlites are key localities for characterizing the relationship between episodic diamond genesis and Kaapvaal craton evolution. Kimberley has 3.2 Ga harzburgitic diamonds associated with creation of the western Kaapvaal cratonic nucleus, and 2.9 Ga eclogitic diamonds resulting from its accretion to the eastern Kaapvaal. Jwaneng has two main eclogitic diamond generations (2.9 and 1.5 Ga) reflecting both stabilization and subsequent modification of the craton. Premier has 1.9 Ga lherzolitic diamonds that postdate Bushveld–Molopo magmatism (but whose precursors have Archean Sm–Nd model ages), as well as 1.2 Ga eclogitic diamonds. Thus, Jwaneng provides the overlap between the dominantly Archean vs. Proterozoic diamond formation evident in the Kimberley and Premier diamond suites, respectively. In addition, the 1.5 Ga Jwaneng eclogitic diamond generation is represented by both sulfide and silicate inclusions, allowing for characterization of secular trends in diamond type and composition. Results for Jwaneng and Kimberley eclogitic sulfides indicate that Ni- and Os-rich end members are more common in Archean diamonds compared to Proterozoic diamonds. Similarly, published data for Kimberley and Premier peridotitic silicates show that Ca-rich (lherzolitic) end members are more likely to be found in Proterozoic diamonds than Archean diamonds. Thus, the available diamond distribution, composition, and age data support a multistage process to create, stabilize, and modify Archean craton keels on a billion-year time scale and global basis.